The technology for manufacturing circuits comprising components mounted on boards has evolved continuously during the past fifty years keeping pace with the evolution of discrete semiconductor devices to present high density integrated circuits on a chip.
In the earliest techniques for the manufacture of PCBs, components were hand soldered onto the board. The next evolutionary step involved exposing the unsupported PCB to a solder wave. The problem with this technique was that the trailing and leading sides of the PCB had no support, thus causing the middle of the board to bow in the middle from the front to the back. This led to the technique of laying the PCB on a fixture which was a pallet having a "window" opening. When double sided PCBs were introduced having components on both sides, the "selective wave pallet was introduced which was designed to cover and protect components on the bottom of the PCB and allow the solder wave to enter cavities that are milled in the board. Registration of the PCB with the pallet was maintained by milling a depresseion in the pallet having the outline of the PCB so that the board was laid in the depression. Previous technology also used tooling pins for registration in some cases.
With the demand for greater density of components (IC's), exposure of the PCB to the heat of a solder wave at 450.degree. F. became a more important issue. This led to the development of the "mask wave" pallet which was used even though there were no components on the bottom of the PCB. This pallet was a full sheet with only holes corresponding to pin through locations permitting exposure to the solder wave.
Pallets according to present practice combine all of the features of the window frame pallet, the selective pallet, and the masking pallet. All of these techniques, used separately or in combination, present limitations to the continued demand for greater component density and economy of manufacture. These techniques include the machining of pockets in the pallet for nesting the PCB which must be very accurate in order to maintain accurate registration of the PCB with the pallet. Even with accurate machining of the pockets, differences in heat expansion between the pallet and PCB complicate the problem of maintaining registration. The machining of pockets requires that the pallet sheet have thickness that is greater than a minimum thickness necessary for the machining operation. The amount of heat absorbed by the pallet is proportional to the thickness of the pallet and the efficiency of the soldering process is diminished by increased absorbtion of heat. Therefore, common practice is select a pallet thickness depending on PCB design.
Variation of the thickness of the pallet requires that the height of the crest of the solder wave must be adjusted according to the thickness of the pallet. The distance of the PCB surface to the crest of the solder wave must be set very accurately in order that the wave barely "brush" the surface of the PCB. Typically, the accuracy of this adjustment is set equal to one half the thickness of the PCB. Present procedure is to support a PCB on a pallet over the molten solder pool and adjust the height of the surface of the pool to carry out the wave solder step. This procedure is particularly costly because it is a "trial and error" procedure and usually several test "runs" must be performed as a part of the adjustment procedure.
A number of disclosures have appeared related to fixtures for manufacturing of printed circuit boards.
For example in surface mount technology, U.S. Pat. No. 5,785,307 to Chung discloses a frame for supporting a printed circuit board. Spring loaded clamps are mounted around the area with fingers that may be oriented out over the area to retain the board. The board is released when the clamp is rotated so as to orient the finger out of contact with the board. Japanese Patent 48238 to Seisakusho discloses a one touch security arrangement for securing a PCB to a table.
None of these inventions disclose a pallet for a PCB that overcome all of the problems of registration, minimizing warpage due to exposure to heat, components lifted off the board during wave solder and, frequent adjustment of the crest of the solder wave.